1. Field of the Invention
The present invention relates to magnetic switch assemblies for detecting unauthorized entry through doors or windows. More particularly, the invention relates to such a switch assembly that more effectively defeats magnetic manipulation by an intruder's magnet.
2. Description of the Prior Art
Magnetic switch assemblies for use with alarm systems are known in the art. One common switch assembly includes a magnetic reed switch mounted in a door or window frame and a magnet carried by the adjacent openable door or window. The magnet holds the reed switch in its opened or closed position (depending on whether the switch is of the normally opened or normally closed type) when the door or window is closed, and permits the reed switch to switch to its other position when the door or window is opened. The reed switch is typically interposed in an electrical circuit of an alarm system so that upon unauthorized opening of the door or window, the switch operation generates an alarm signal.
Unfortunately such reed-type switch assemblies can be readily defeated by simply placing an external magnet adjacent the door or window frame in proximity to the reed switch. The external magnet holds the reed switch in its non-alarm position and thus allows the door or window to be opened without triggering the alarm.
Improved magnetic switch assemblies have been developed to prevent such magnetic defeating of an alarm. For example, U.S. Pat. Nos. 5,332,992 and 5,530,428, hereby incorporated by reference, each disclose a magnetic switch assembly including a ball-type switch that is normally mounted in the door or window frame and magnet that is normally mounted in the corresponding door or window. The ball switch is held in one switch position by the door magnet and is shifted to the other switch position whenever the door is opened or when an intruder's magnet is placed in the vicinity of the switch assembly. This allows the switch to both detect the opening of the door or window and to prevent magnetic manipulation of the switch by an intruder's magnet.
Applicant has discovered that the switch assemblies disclosed in the '992 and '428 patents can be disarmed under certain circumstances. Specifically, when an alarm system is temporarily turned off to allow a person to pass through a door, the person could secure a thin magnet directly under the ball switch in the door frame to emulate the door magnet. The magnet would fit between the space between the door and frame and therefore be concealed from view.
Thereafter, when the alarm system is turned back on, the magnet will hold the ball switch in its non-alarmed position regardless of the position of the door. Thus, the intruder's magnet would permit the door to be opened without triggering the alarm.
An intruder could therefore enter a house or building under false pretenses during the day when the alarm system is off and disarm the switch assembly so that he or she could later return undetected. Those skilled in the art will appreciate that although low-security installations would not likely be subject to such sophisticated attempts of entry, high-security installations such as military, government and business buildings could be.
Prior art magnetic switch assemblies also cannot be tested from a central control center, but instead must be individually and manually tested by moving their associated doors or windows while their alarm systems are on to verify that the switches move to their alarmed states. Those skilled in the art will appreciate that such manual testing is time consuming, especially in large installations having many switch assemblies.
Another limitation of many prior art magnetic switch assemblies is that they must be formed with a sloped lower surface or include a small spring between the ball and one of the switch elements to bias or push the ball from its normally opened or closed position when the door or window is opened. Such biasing techniques increase the cost and complexity of switch assemblies and require more precise alignment of the switches in their door or window frame during installation. Moreover, such biasing techniques occasionally fail to bias a ball off its normally opened or closed position when its door is opened because of arc welds formed between the ball and one or both of the switch elements. Thus, prior art magnetic switches occasionally fail to detect the opening of the door or window and thus fail to trigger an alarm.